1. Field of the Invention
The present invention relates to a piezoelectric or voltage-controlled oscillator having a semiconductor integrated circuit and a piezoelectric resonator. The present invention relates to a method of producing the piezoelectric or voltage-controlled oscillator.
2. Description of Related Art
In recent years, great reductions in size and weight have been achieved in various information equipment including HDDs (hard disk drive), portable computers and mobile communication systems such as portable or mobile telephones. As a result, the size and thickness of piezoelectric and voltage-controlled oscillators for use in these devices must be reduced. Thus, there is a need for piezoelectric and voltage-controlled oscillators of a surface mounting type that can be mounted on either surface of a circuit board. FIGS. 19a and 19b illustrate an example of a Conventional piezoelectric oscillator employing a quartz-crystal resonator as a piezoelectric resonator. FIGS. 20a and 20b illustrate an example of a conventional voltage-controlled (crystal) oscillator (VCXO).
In the conventional quartz-crystal oscillator shown in FIGS. 19a and 19b, a CMOS IC chip 101 is mounted by a conductive adhesive on an island 103 that is a part of a lead frame 102. The IC chip 101 is electrically connected to input/output lead terminals 105 by Au (wire-bonded) wires 104. A quartz-crystal resonator 106 is fixed to inner leads 107. As shown in FIG. 19b, the quartz-crystal resonator 106 includes a cylinder shaped quartz chip having a circular cross-section with a diameter of about 3 mm. The quartz-crystal resonator 106 is electrically connected to the gate electrode 108 and the drain electrode 109 of the IC chip 101. The IC chip 101, the quartz-crystal resonator 106 and parts of input/output lead terminals 105 are molded by transfer molding with an epoxy resin molding resin to form a plastic package 110 of the quartz-crystal oscillator as best shown in FIG. 19b.
In the conventional voltage-controlled oscillator shown in FIGS. 20a and 20b, electric circuit components such as a transistor 111 and variable-capacitance diode 112 are mounted on a substrate 113 that is fixed by solder on the stem 114 of a metal can package 116. A quartz-crystal resonator 115 is also mounted on the substrate 113. The can 116 is hermetically sealed by resistance welding for example. In another common type, a trimmer capacitor or the like is provided on the substrate 113 and an adjusting hole is formed in the can 116 so that the frequency may be adjusted after the voltage-controlled oscillator is mounted on a circuit board installed in a device such as a mobile communication system.
In the conventional piezoelectric and voltage-controlled oscillators described above, the piezoelectric resonator is accommodated in a cylinder case having a diameter of about 3 mm. As a result, the piezoelectric or voltage-controlled oscillator has a large height such as about 4.5 mm to 7 mm. Thus, its total volume is as great as 0.5 cc to 1.0 cc. The conventional piezoelectric and voltage-controlled oscillators of this type cannot meet the small-size requirements that are essential in small-sized electronic devices such as HDDs portable computers or portable/mobile telephones.
To reduce the thickness of the plastic-molded piezoelectric or voltage-controlled oscillator, the thickness of the piezoelectric resonator must be reduced. There are two known techniques for this purpose.
A first technique is to use a piezoelectric element having a smaller size and dispose it in a cylinder case having a smaller diameter. If the diameter of the piezoelectric resonator is reduced to 1.5 mm, its size will be 0.5 to 0.7 mm (W).times.5.6 mm (L) which is much smaller than a typical size of 1.8 to 2.0 mm (W).times.5.6 mm (L) of a piezoelectric element disposed in a case having a diameter of 3 mm. However, there is difficulty in designing and producing a piezoelectric element (such as a quartz chip) having such a small size while maintaining the required performance. This technique is therefore expensive and impractical.
A second technique is to reduce the thickness by forming the cross-section of the piezoelectric resonator into an elliptical shape or a track shape (combination of a rectangle and two semicircles). In this technique, the piezoelectric element disposed in the case is allowed to have a size similar to that of the conventional piezoelectric element. Therefore, it is possible to produce a piezoelectric element without increasing the cost.
An example of a piezoelectric oscillator produced using the second technique is disclosed in Japanese Patent Laid-Open No. 4-259104 (1992), the subject matter of which is incorporated herein by reference. In this example, the small-sized oscillator employs a piezoelectric resonator whose cross-section is flat at one end.
When a piezoelectric or voltage-controlled oscillator is molded with resin, projection pressure is uniformly imposed on the piezoelectric resonator and semiconductor integrated circuit disposed in the oscillator.
A hollow case is used with the piezoelectric element disposed therein. Therefore, the case has mechanical strength strong enough not to be deformed by pressure during the molding process so that the piezoelectric element does not come into contact with the inner walls of the case and no strain occurs either in the piezoelectric element or in the mounting part.
Deformation also depends on the shape of the piezoelectric resonator. Computer simulation using structure analysis software has revealed that when using a piezoelectric resonator having an elliptic or track-like cross-section, non-uniform deformation is induced by uniform pressure arising during the molding process. Greater deformation occurs in parts of the case parallel to the major axis of the ellipse or in linear parts of the track shape compared to the cylinder type in which deformation occurs uniformly. Similar results have been observed in experiments on the effects of molding.
Deformation of the case caused by the shrinkage of the molding resin also depends on the location at which the piezoelectric resonator is disposed in the piezoelectric or voltage-controlled oscillator. That is, when the molding resin shrinks and thermal stress occurs in the molding resin, thermal stress varies from part to part of the piezoelectric or voltage-controlled oscillator. Therefore, the location of the piezoelectric resonator and structure of the piezoelectric or voltage-controlled oscillator must be optimized.
In the example shown in Japanese Patent Laid-Open No. 4-259104 (1992), an IC chip is attached to a flat part of the cross-section of the piezoelectric resonator. The piezoelectric resonator and the IC chip are then disposed and molded in a substantially central part of a piezoelectric oscillator with a resin.
However, the area of the flat part must be greater than the area of the IC in order to attach the IC on the flat part of the piezoelectric resonator. Therefore, the area of the flat part must be increased with an increase in the size of the IC chip. To increase the area of the flat part of the piezoelectric resonator, the length of the linear part of the track shape must also be increased. This results in an increase in the pressure applied to the piezoelectric resonator during molding which in turn increases the deformation of the case.
Since the thickness of the resin disposed on the piezoelectric resonator is greater near the IC chip than in an opposite part, the part near the IC chip has greater deformation compared to the opposite part.
As described above, to achieve a reduction in the thickness of a piezoelectric or voltage-controlled oscillator employing a piezoelectric resonator having an elliptic or track-like cross-section, the shape of the cross-section of the piezoelectric resonator and the position at which the piezoelectric resonator is disposed must be optimized.
There is also a need for higher accuracy in the oscillation frequency of piezoelectric and voltage-controlled oscillators.